1. Technical Field
The invention relates in general to a tuner and a gain adjustment method thereof.
2. Background
The orthogonal frequency division multiplexing (OFDM) modulation technology can be used in digital video broadcasting systems to perform high spectral efficiency transmission. Based on the OFDM modulation technology for the multi-carrier modulation, the effective processing ability is obtained with respect to the multi-path reflection effect. So, the OFDM system has recently become the mainstream technology in the digital video broadcasting application development. Various systems, such as the Digital Video Broadcasting-Terrestrial (DVB-T), the Integrated Service Digital Broadcasting-Terrestrial (ISDB-T) and the China Mobile Multimedia Broadcasting (CMMB), use the OFDM transmission technology.
To provide desirable quality of video, all digital video systems have stringent requirements on adjacent channel interference rejection and sensitivity. To meet the stringent requirements, RF receivers are designed to have good noise figure (NF), third-order intercept point (IIP3) and 1-dB compression point (P1 dB). However, there are trade-offs among the above-mentioned figures of merits related to the gain of the RF front-end circuits such as a low noise amplifier (LNA). For example, as the gain increases, NF improves but the linearity (IIP3 and P1 dB) gets worse and vice versa. Hence, automatic gain control mechanism is proposed to adjust the gain of the RF front-end circuits appropriately.
A timing based LNA gain adjustment in an RF receiver, as shown in FIG. 1, is disclosed in U.S. Pat. No. 7,242,915 B2. The LNA gain adjustment circuit executes the gain tuning mechanism at guard periods without read data. The above-mentioned method may be applicable for the Global System for Mobile Communication (GSM) with no strong adjacent channel interference, however, the method with no gain adjustment during data frames is not suitable for digital TV systems with strong adjacent channel interference and rapidly varying signal power. In addition, the mixer may filter out the far away interference, so that the received signal strength indicator (RSSI_A) can no longer accurately represent the influence of the interference and is unable to adjust the gain of the LNA appropriately.